Method of producing thread swaging devices



y 1965 H F. PHIPARD, JR 7 3,195,156

METHOD OF PRODUCING THREAD SWAGING DEVICES Filed June 27, 1961 9 Sheets-Sheet 1 20 Fig. 2

Fig. 3 Fig. 4 39 Fig. 5

IN V EN TOR.

HARVEY F PHIPARD, JR.

BUCKHORN, CHEATHAM a BLORE ATTORNEYS July 20, 1965 3,195,156

METHOD OF PRODUCING THREAD SWAGING DEVICES H. F. PHIPARD, JR

"9 Sheets -Sheet 2 Filed June 27, 1961 Fig. 8

INVEN TOR.

HARVEY F. PHIPARD, JR.

BUCKHORN, CHEATHAM 8 BLORE I ATTORNEYS July 20, 1965 H. F. PHIPARD, JR 3,195,156

METHOD OF PRODUCING THREAD SWAGING DEVICES 9 Sheets-Sheet 3 Filed June 27, 1961 Fig. 9

O i w a a Q a 7 3 F5? 2 a qllllllllllo a m B O 6 5 x E 0 a 0 p A J m I 2 6 LEIIri Fig. IO

INVENTOK HARVEY E PHIPARD, JR.

BUCKHORN, CHEATHAM a BLORE ATTORNEYS y 1965 H. F. PHIPARD, JR 3,

METHOD OF PRODUCING THREAD SWAGING DEVICES Filed June 27. 1961 9 Sheets-Sheet 4 INVEN TOR. HARVEY F. PHIPARD, JR.

BUCKHORN, CHEATHAM 8 BLORE ATTORNEYS y 1965 H. F. PHIPARD, JR 3,195,E

METHOD OF PRODUCING THREAD SWAGING DEVICES Filed June 27, 1961 9 Sheets-Sheet 5 Fig. 15

INVENTOR.

HARVEY E PHIPARD JP EUCKHORN; Sh'EIETf-MA'F 8. BLOHE XlT'f'ORNEYS y 165 H. F. PHIPARD, JR 3 W5 fl5$ METHOD OF PRODUCING THREAD SWAGING DEVICES Filed June 27, 1961 9 Sheets-Shem 6 Fig. I? 98 IN V EN TOR.

HA RVE Y F PHI PARD, JR.

' BUCKHORN, cw mmm a ELORE ATTORNEYS July 20, 1965 F. PHlPARD, JR

METHOD OF PRODUCING THREAD SWAGING DEVICES Filed June 27. 1961 9 Sheets-. Sheet 7 m .m F

IN VEN TOR. HARVEY F. PHIPARQ (1F? BUCKHORN, CHEATHAM 8: BLORE AT TORNEYS y 20, 1955 H. F. PHIPARD, JR 3,395,15fi

METHOD OF PRODUCING THREAD SWAGING DEVICES Filed June 2'7. 1961 9 Sheets-Sheet 8 IN VEN TOR. HARVEY F. PH IPA RD, JR.

BUCKHORN, CHEATHAM a BLORE ATTORNEYS July 20, 1965 H. F. PHIPARD, JR 3,

METHOD OF PRODUCING THREAD SWAGING DEVICES Filed June 27, 1961 9 Sheets-Sheet 9 Fig. 27 q Fig 2a U r Fig. 29

Fig. 3/

\y INVENTOR.

HARVEY F. PH/PARLZ JR.

BUCK/109M BLORE, KLAROU/ST 8 SPAR/(MAN United States Patent 0 3,195,156 METHQED 0F PRGIBEICENQ THREAD SWAGENG DEWCES Harvey F. ihipard, .lr., South Dartmouth, Mesa, assignor to Research Engineering and lviauutacturing, End, New

Bedtcrd, Mass, a corporation of Massachusetts Filed June 27, 1961,, Sea. No. 115,846 16 Qlaims. (will. 1tl-1l) The present invention relates to vices, particularly to self-tapping screws and the like devices, and to factoring the same.

This application is a continuation-in-part of my copending applications, Serial No. 819,167, filed June 9, 1959 and Serial No. 22,491, filed April 15, 1960, both of which are now abandoned.

Self-tapping screws fall into two broad classes, the first being those which are provided with cutting edges at the work-entering end, the second, and most common, being those which are so designed as to form threads with a swaging operation. Screws of the first type have numerous disadvantages, one of the most significant being that they all form chips which are cut from the body into which they are driven. While screws of the second type form no chips, they have other equally serious limitations. Depending upon the nature and hardness of the metal into which they are driven, screws of the second type require a high driving torque, particularly in metal greater than one-eighth inch in thickness. High driving torque is objectionable, not only as regards manual drivability, but also in connection with the use of clutch controlled power driver-s such as are used in assembly lines. The driver clutch must be so set that the screws will consistently be driven home to the fully seated position before disengagement. However, as is well known, the driving torque of individual screws varies considerably due to presence of any lubricant, surface condition of the threads, and other variable factors. Similarly, the stripping torque of the mating threads as well as the failure torque of the screws vary considerably from one to the next. Also, clutch mechanisms of the power drivers cannot be relied upon to disengage at precisel the same torque value each time. Therefore, if the differential between the average value of the driving torque of a quantity of screws and the average value of the failure torque is relatively narrow, it will be extremely difficult to adjust the clutch so that the driver will be disengaged properly each time. When this does not occur, the threads will be stripped, or the screw will be broken, either of which will result in costly delays of the assembly line while repair or replacement is made.

While the disadvantage of high driving torque as compared with the failure torque is particularly true with respect to the ASA conventional types A, B and C screws, when used with thick metals, it is also true to a greater or lesser extent with respect to the various types of the thread-cutting screws, especially so in such instances where it is attempted to form threads in a thick body member to substantially full thread depth.

It is a primary object of the present invention, therefore, to provide a new and improved self-threading member of the thread-forming or swaging type which forms threads in the parent body without producing any chips, which has a substantially lower driving torque than comparable thread-forming devices available heretofore.

A further object of the invention is to provide a new and improved thread-forming screw having a relatively low driving torque and a relatively high stripping or failure torque so that the difierential between such torques is relatively great, making it easier for the adjustment of thread-forming deor thread-forming methods for manu- 3,i5,l5h Patented July 20, 1%65 ice automatic clutches of power drivers so that clutch disengagement can more readily be eflected prior to failure of the screw or stripping of the threads of the parent body.

- A still further object of the present invention is to provide a new and improved self-threading device whereby substantially full threads may be formed with a substantiaiiy perfect fit thereby resulting in a more perfect assembly due to the snug fit between the screw and the parent body.

A still further object of the present invention is to provide a new and improved self-threading device which may be driven into metal of any thickness, such as, into blind holes.

The above mentioned objects are accomplished by making the thread-forming device of arcuate polygonal cross-sectional shape having an odd number of sides. While others have heretofore proposed thread-forming devices of various different noncircular cross-sectional shapes, those have been of such a complicated nature as to render them entirely unsuitable for production on a commercial basis, or else the cost of manufacture was so great as to restrict the field of use within very narrow limits. In most instances, however, such non-circular, or lobular, cross-sectional shapes as have been proposed heretofore were accompanied by one or more serious defects, either in details of configuration or in the methods required for manufacturing the same, so as substantially to preclude their successful manufacture and usage. The particular noncircular, or lobular shapes herein disclosed as being of my invention, are such as enables the manufacture of the thread-forming devices on mass production basis with machinery readily available .while the resultant product possesses the superior performance characteristics recited above.

Another object of the invention is to provide a new and improved method of making a lobular thread-forming device, which method resides in forming a blank having a work-entering end portion, shaping at least such portion into an arcuate lobular configuration having an odd number of arcuate sides with each transverse cross section of substantially equal width throughout 360 degrees, and then rolling a continuous thread on the blank including the end portion, and while doing so, maintaining on at least the end portion an arcuate lobular pitch surface configuration that in every transverse cross section is of equal width throughout 360 degrees, and during the rolling operation forming a taper on the crest of the thread on the end portion, which taper extends inwardly toward the tip thereof.

For a consideration of what I believe to be novel and inventive, attention is directed to the following disclosure while the invention itself is pointed out with greater particularity in the appended claims.

In the drawings:

FIG. 1 is a side view of a thread-forming member according to one form of the present invention;

FIG. 2 is an end view of the member shown in FIG. 1;

FIG. 3 is a side view of a blank used for making the screw shown in FIG. 1;

FIG. 4 is an end View of blank shown in FIG. 3;

FIG. 5 is a schematic view illustrating the rolling of screw blanks between a pair of fiat faced thread rolling cues;

FIG. 6 is a schematic view illustrating a rotary thread roller;

PEG. 7 is a view, partly in section, illustrating a screw driven into a body of metal;

FIG. 8 is a diagram illustrating one convolution of a thread such as along the line 88 of FIG. 1;

FIG. 9 is a planar projection of the spiral curve illustrated in FIG. 8;

FIG. 1.) cross section of certain threaded members herein dis- FIG. is a view, somewhat schematic, taken along the line 1010 of FIG. 7;

FIG. 11 is a view taken along the line 1111 of FIG. 10; 7

FIG. 12 isa view taken along the line 12-12 of FIG. 10;

FIG. 13 is a view taken along the line 12-12 of FIG. 10, but illustrating the screw in the loaded condition;

FIG. 14 is a diagram illustrating the outline of one form of blank;

FIG. 15 is a sectional view of a screw made from a blank of a shape shown in FIG. 14;

7 FIG. 16 is a sectional view taken along the line 16-16 of FIG. 15;

FIGS. 17, 18, 19 and 20'are diagrams illustrating the outlines of blank cross-sections according to various 'different further modifications;

FIGS. 21 to 26, inclusive, are side and end views of different additional modifications of thread forming screws constructed in accordance with the present invention. p

5 FIG. 27'is a view partly in section illustrating a length of Wire being drawn through a re-forming die;

-FIG. 28 is a sectional view taken along the line 2828 of FIG. 7;

FIG. 29 is a view partly in section illustrating the cut off die station of a conventional cold header machine;

FIG. 30 is a view partly in section illustrating the first, upsetting, station of a cold header machine;

FIG. 31 is a View partly in section illustrating the final heading station of a cold header machine.

In this application, the following definitions shall be applicable:

Pitch diameter is used as a generic term to designate the diameter i.e., maximum transverse width, of any section of either the pitch cylinder or the pitch cone as determined by the three-wire method of pitch diameter measurement. (See 58, FIG. 10.) Due to the poly o'nal'nature of certain configurations herein discussed, the pitch diameter, i.e., maximum transverse width, does not always extend throughthe axial center of the configuration. V

Pitch cylinder is, on a straight thread, an imaginary coaxial'cylinder, the surface of which would pass through thethread profiles, or the projection thereof, at such points as to make the width of the groove, or the projection thereon-equal to one-half the basic pitch. (See 30, Due to the. polygonal configuration of the cussed, the pitch cylinder thereof is not round but of arcuate polygonal cross-sectional shape.

Pitch cone on a taper thread, is an imaginary coaxial cone, th e surface of which wouldpass through the thread profiles, or the projection thereof, at such points as to make the width of the groove, or the projection thereof, equal to one-half the basic pitch. (See 26, FIG. 1).

' Pitch diameter cross section as used herein means the transverse cross section of either a pitch cylinder or a pitch. cone, and for reasons-pointed out above it may be of arcuate polygonal configuration. V

Pitch surface is used herein to designate the surface of revolution of the pitch line defining either the pitch cylinder or the pitch cone as hereinbefore defined.

Pitch surface 'cross section is used herein to designate the transverse cross section of any pitch surface, such as that of either the pitch cylinder or the pitch cone, as hereinbefore defined. For reasons mentioned above, it may be of arcuate polygonal configuration.

The terms pitch diameter cross section and pitch surface cross section are intended to be synonymous expressions and are used interchangeably herein.

The three-wire method of pitch diameter measurement is well known in the art and is described, for example,

(g in the US. National Bureau of Standards Handbook No. H28 (1957), and further described below.

Referring now to the view of FIG. 1, the invention will first be described with reference to a screw indicated generally at 20 having a driving head 21 and a straight thread formation 22 on the main shank portion 23 and a tapered thread formation 24 on the work-entering end 25 of the screw. Referring now to the end View of FIG. 2, it will be observed that the threaded portion of the screw is of noncircular, or lobular, shape and which may, for convenience, be termed an arcuate polygon, or more specifically, in this instance, an arcuate equilateral triangle. The configuration is characterized by tree equally spaced lobes 2'7, 28 and 29, having a radius of curvature substantially less than one-half the pitch diameter of the screw, the lobes being separated by relatively broad arcuate sides 31, 32 and 33, each having a radius of curvature substantially greater than one-half the pitch diameter ,of the screw. The sides 31, 32 and 33 merge smoothly and continuously with the intervening lobes, 27, 28 and 2?, respectively.

Referring now to FIGS. 3 and 4, there is illustrated a blank which is preformed for use in the manufacture of the screw illustrated in FIGS. 1 and 2. This blank includes a straight shank portion'35 having a driving head 36 at one end, which head may be provided with any driving means as desired, and 'a tapered end 33 at the other. It will be obvious that the driving end need not necessarily include an enlarged head, inasmuch as otherforms of driving heads are well known. As illustrated in the end view of FIG. 4, the blank is also of arcuate triangular shape similar to the configuration illustrated in FIG. 2. As will be evident from FIGS, 3 and 4, the blank the-rein shown is made by working an odd number of uniformly spaced apart surface areas 37 extending longitudinally of a length of metal stock inwardly of such length to a greater extent than the intermediate surface areas 39 and thereby producing a shank portion that is of generally arcuate polygonal cross section of substantially uniform width throughout 360 degrees. As

will be explained more fully hereinafter, the difference in the radial distances from the midpoint of the areas 37 and 3? to the central axis 45 of the blank is not substantially. more than two-thirds and not substantially less than one-quarter the depth of the thread. Referring now to FIGS. 27 to 31, inclusive, the blank can be formed from stock prepared by drawing a length ofround wire 16% through. a forming or sizing die 162 provided with a triangularly shaped orifice 164 of a size and shape desired for the cross section of the blank as illustrated in FIG. 4. Stated in another Way, as the round wire is drawn through the die162 the fiow of metal is so con trolled by the die orifice so' as to form the cross section of the Wire with an odd number of symmetrically arranged, arcuate sides. Stock material 166 so formed can then be fed into a conventional cold heading machine 168 wherein predetermined length-s may be severed by cutoff means 179 and an end portion upset and headed with conventional upsetting and heading punches 171 and 1'72. The heading punch 172 can provide either a slot, recess or other driving means such as a hex head, as may be desired, in the upset head portion. During the heading operation, a taper 38 may simultaneously be placed on the end of the blank in any manner well known in the art such'as, for example, by providing an inwardly tapering end 174 in the cavity of the heading die 171:, in the event that such a taper i desired in the final article. The form of screw illustrated in FIGS, 25 and 26, for example, does not require any such taper, however.

It will be obviousthat in forming a taper 33 during the heading operation, the work-entering end of the shank portion 35 is circumferentially reworked gradually and increasingly inwardly in the direction toward, the extremity of the end portion while maintaining an arcuate polygonal cross-sectional configuration substantially concentric and symmetrical with that of the shank portion as shown most clearly in the end view of FIG.

With reference to FIG. 4, it will be observed that the transverse width of the blank as determined by a micrometer, is substantially constant or equal throughout 360 around the blank even though it is not round so that a thread may be generated thereon with readily available centerless thread-forming machines. The blank 35 may be threaded between conventional flat thread-rolling dies 49, 41 as illustrated in MG. 5. Alternatively, the blank 35 may have threads formed thereon as by rolling it between a rotating die 43 and a stationary arcuate die 44- as shown in FIG. 6. The various thread-rolling machines indicated in F368. and 6 are characterized by the fact that they all form threads on a cente-rless basis. Moreover, the opposed workpiece supporting surfaces are spaced a substantially uniform distance apart throughout the thread-forming operation and which is important for ease and economy of manufacture. It is to be understood, however, that some forms of conventional threadforrning means referred to may be contoured to form certain types of taper threads on the work-entering end, and are also slightly and uniformly inclined toward each other from the starting to the finish ends. During the thread-rolling operation, a continuous thread is formed on both the shank and tapered end portions 35 and 33, respectively, of the blank. During the thread-rolling operation, the thread, particularly on the end portion of the blank, is provided with an arcuate lobular pitch surface configuration, which configuration is substantially the same as that of the corresponding cross sections of the blank, as will be noted by a comparison of FIGS. 2 and 4. Moreover, during the rolling operation, a taper is formed on the crest of the thread into its final shape on the end portion, which taper extends inwardly toward the tip thereof. As will subsequently be pointed out, the taper may be formed on the thread over the work-entering end solely during the rolling operation with conventional contoured dies, and without requiring previous tapering of the end of the blank.

Referring now to the view of PEG. 7 which illustrates the screw 249 being driven through a metal member 51., it will 'be observed that no metal is cut as the threads are formed in the side Walls of the bore 52 in the member 51. Instead of being cut, the metal of the member 51 is displaced by a swaging operation comparable .to that in which threads are formed on a screw when passed between thread-rolling dies ill and 41. This, of course, is true of all self-threading screws of the thread-forming or swaging type, such as, for example, the ASA conventional type C. Screws of the present invention have a lower driving torque than conventional screws of the last mentioned type due to the configuration of the threads over the work-entering end portion and which will now be described with reference to FIG. 8.

In FIG. 8 the line 54 illustrates the peripheral or crest contour of a single spiral revolution of thread on the tapered end 24 of the screw 20. The root of the thread portions which will be formed in a parent body, such as 51, by the crests of the lobes 27a, 28a and 29a, may be represented by the circular arcs 55, 56 and 57, the extent of which may be further represented by the arcs D, E and F. From the crests of the lobes the screw thread recedes from contacting engagement with the surfaces of the thread formed in the parent body and hence there is no frictional contact throughout the entire extent of the arcs D, E and F. The working engagement of the lobes 28a, 29a and 2715 with the body of parent material is indicated by the arcs A, B and C. The dotted circle 8 indicates the root diameter of the thread formed by a complete revolution of the lobe 275.

FIG. 9 illustrates a planar projection of the single spiral revolution illustrated in FIG. 8. The successive thread lobe crests, 27a, 28a, 2-911 and 27b are indicated. The starting radius, that is, the distance from the axis 6%} of the screw to the crest of the lobe 2% is indicated at 61 while the distance from such axis to the crest of the dual lobe 27b is represented at 62. The extent of working engagement of the successive lobes 28a, 29a and 2% with the metal of the parent body may be represented by the distances A, B and C, it will be observed that the total engagement amounts to approximately 25% of the total peripheral extent of the screw thread. Due to the fact that approximately three-fourths of the screw thread is thus held out of engagement with the metal of the parent body, the frictional drag is held to a minimum and the driving torque of the screw is thus held to a minimum. For this reason, the screw as illustrated may readily be driven through relatively thick metal members with very low driving torque requirement. At the same time the an le of inclination of the thread portions over the distances A, B and C is not so steep that they will cut chips from the body being threaded.

In FIG. 10 is illustrated a schematic sectional view through the shank of the screw, and in which the line 64 indicates the peripheral, or crest edge of the screw thread, 65, the root of the thread While the dotted line as represents the pitch cylinder of the thread. Particular attention is directed to the fact that the pitch cylinder is not round but in this instance is of arcuate polygonal crosssectional configuration. The dotted line as is also sometimes referred to hereinafter as representing the pitch surface of the thread inasmuch as considerable of the following description referring to this aspect is applicable to the pitch cone of the thread as Well as to the pitch cylinder there-of. Also the transverse cross section of this pitch surface as is sometimes, for convenience, referred to hereinafter as the pitch diameter cross section of the thread. The line 67 represents the root of the completed thread formed in a parent body such as 51 and which is a true circle concentric With the screw axis 6:). The radius of the circle 67 corresponds with the distance between the screw axis 6t? and the outermost points on the thread lobes Z7, 23 and 29. It will be obvious that only those portions of the screw threads at the outer portions of the lobes 27, 28 and 29 will be in firm contacting engagement with the threads formed in the parent body and which is furthermore indicated in the sectional view of FIG. 11. The relatively broad sides 31, 32 and 33 of the screw thread will be supported out of engagement with the adjacent threads in the parent body 51 as illustrated in FIG. 12 on account of the outof-round condition of the screw. Accordingly, during driving operation, frictional resistance between the screw and the parent body is reduced to a minimum in the shank portion as Well as in the tapered work-entering portion as previously described.

The pitch diameter of the threaded section, or in other words, the diameter of the lobular pitch cylinder, or pitch surface, 65, illustrated in FIG. 10, is represented by the distance 65. This diameter, or distance, as will be explained more fully hereinafter, is substantially constant throughout 360 degrees. The pitch diameter of an arcuate triangular threaded section may be measured by the previously mentioned three-wire method of measurement, as indicated by the wires 46 and 47 in FIGS. 1 and 10. The diameter of these wires is selected so that they will contact the flanks of the threads at the points coinciding with the pitch cylinder indicated at 369 in FIG. 1 and by the line 65 in FIG. 10. In the case of a uniform thread illustrated in FIG. 1, tie contact points will, of course, fall at the midpoints of the sloping thread fiank faces. With a suitable micrometer or other measuring device, having parallel plane gauge surfaces, the distance between the outer surfaces of the oppositely disposed wires may be measured. From this value, the pitch diameter 68 can readily be computed. The pitch diameter for any section of a pitch cone, such as that indicated at as in FIG. 1, may also be measured similarly by the three-wire method taking into consideration, of course, the angle of taper of the cone at the seC- tion 'being measured. I

It will be obvious that when viewed in FIG. 10, the line of measurement of the maximum transverse dimension, or pitch diameter, 68 of the lobular pitch cylinder 66 from point G to point H passes through the center 60. If, however, the threaded section is rotated slightly in either direction relative to the wires 46 and 47, the distance 68 will remain the same, but the line of measurement, that is the pitch diameter, say from point L to point M, for example, will not pass through the center 60 as will be explained more fully hereinafter. It is to be understood, therefore, that in stating that the pitch diameter of either the pitch cylinder or the pitch cone is uniform or equal throughout 360 degrees in any cross section, the line of such measurement does not necessarily extend through the center of the screw as in the case of a true diameter. Another way of stating this condition would be to say that every transverse cross section of the lobular pitch surface of the thread, that is, of either the pitch cylinder or the pitch cone, as the case might be, is of equal transverse width throughout 360 degrees.

From the brief description given thus far, it will be apparent that the thread-forming devices of the present invention are susceptible of considerable variation insofar as the cross-sectional configuration is concerned. Certain basic considerations must be observed, however, in order that the blanks may readily be threaded by available, conventional thread-rolling equipment as previously referred to. To begin with, at least that portion of the blank which is to be provided with lobular threads, as herein disclosed, must be of such cross-sectional shape that it will readily rotate between the opposed threadforming dies. This means that the blank portion should be of substantially uniform width throughout 360. By the term uniform width, it is meant that the crosssection of the blank portion is of'uniform transverse width as determined by a micrometer even though such cross-section is not round. It is to be understood, of course, that this is' not critical inasmuch as transverse starting lines may be provided upon the die faces for initially starting the rotation of the blank, and once the blank rotation is started, the frictional engagement between the blank and the dies may be relied upon for maintaining the rotation of the blank throughout the thread-forming operation.

Referring now to FIG. 14, an outline of a blank is shown in its simplest form, namely, in the shape of an arcuate, equilateral triangle 72. Each side of this triangle is arcuate with respect to the intersection of the other two sides as indicated by the radius 73. It will be obvious that the triangle 72 is of uniform width throughout 360 and hence may be freely rotated between two parallel surfaces spaced apart a distance corresponding to the length of the radius 73.

If a blank having a shape corresponding to the triangle 72 were passed between a pair of flat thread-rolling dies,

vit would be obvious that a considerable deformation would occur on account of the fact that the threads of the 4 dies would penetrate more readily and to a greater depth along the ridges of the blank than on the opposite sides with the result that the threads would be completely formed along the ridge or lobe portions of the blank and would probably be unfinished along the directly opposite portions. Moreover, the ridges or lobe portions of the blank would be considerably rounded instead of being defined by a sharp line as in the original blank.

In FIG. is illustrated an approximaiton of the crosssectional shape of the resultant screw after a blank of a shape such as in FIG. 14 has been passed between a pair of flat rolling dies. It will be observed that the lobes 78, 79 and 36 are smoothly rounded with a curvature which merges with the broad sides 82, 83 and 84. On account of the relatively large amount of out-of-round with respect to the blank, as indicated by the distance 74 in 1 16.14,

a similar, though somewhat lesser, condition willexist with respect to the finished screw 77. As illustrated by the view of FIG. 16, the threads of the screw 77 are spaced considerably from the threads of the parent body 51 in the region of the broad sides'of the screw. It will be obvious, therefore, that while the screw 77 will have a Very low driving torque, the stripping torque will also be lower than desired for most applications. In other words, upon application of an end-wise or axial thrust between the screw 77 and the body 51, such thrust will be resisted only by the interengagement of the lobular portions 73, 7h and 81) of the screw and the adjacent surfaces of the threads of the member 51. Consequently, it will be apparent that stripping of the threads of either the screw or the parent'body will occur at a relatively low stress value. For certain applications, however, such as where high stripping strength is not required, such as in the case of a screw which is to be subjected only to sheer forces, a cross-sectional configuration comparable to that illustrated in FIG. 15 may be entirely satisfactory. As previously pointed out, in rolling threads upon a blank having a cross-sectional shape as indicated by the triangle 72 in FIG. 14, the threads might'not be fully formed in the broad sides 82, 83 and $4. In view of the fact that the 7 threads in such regions are not relied upon for any particular purpose, as previously mentioned in connection with FIG. 15, such imperfections may be of no consequence in this particular case. For applications where considerable stripping strength is required, this may be accomplished by reducing the amount of clearance between the threads in the area between the lobes and the adjacent threads of the body member to a smaller amount such as to a matter of a few thousandths of an inch as illustrated in the view of FIG. .12. As will subsequently be pointed out,such reduction in the amount of clearance may be done, according to the present invention, without greatly increasing the driving torque. I

In FIG. 17 is illustrated at 9% a further outline configuration for a screw blank in the form of another arcuate equilateral triangle having sides 91,92 and 93, each side having a radius of curvature 94, somewhat greater than one-half the diameter of the circumscribed circle 95.

sponding to the arcuate triangle is rolled between a pair of flat thread-rolling dies, the lobes 98, 99 and 100 will be rounded during the thread-rolling operation so that the resultant screw will have a cross-sectional shape diflering slightly from that illustrated in FIG. 17 and more nearly approximating that illustrated in FIG. 18.

Referring now to FIG. 18, the arcuate triangle therein illustrated comprises broad sides 102, 103 and 104 having a radius of curvature 165. These sides merge'smoothly and tangentially with intermediate arcuate lobes 1%, 109 and 1163 each having a radius of curvature 111. The lobes are internally tangential with respect to the circumscribed circle 114. The maximum clearance between the sides 102, 1&3 and 1M and the circumscribed circle 114 may be indicated by the distance 116. a

A blank having a cross-sectional configuration comparable to that illustrated in FIG. 18 will roll smoothly between a pair of flat rolling dies in that the width is substantially uniform throughout 360. Moreover, with an amount of clearance as indicated at 116, threads can readily be completely formed over the sides 162, 103 and 104 as well a across the lobes, 1%, 109 and 110. Moreover a minimum of deformation will occur in cross sectional shape between that shown in FIG. 18 for the blank and that of the finished screw. This is a distinct advantage insofar as commercial practice is concerned since it is thus possible to predict and thus design in advance, within reasonably close limits the final cross-sectional shape of the screw by corresponding design of the blank.

In the case of rolled threads, the volume of metal which will be raised above the surface of the blank will correspond to the volume of the grooves formed in the blank between the raised thread portions. Accordingly, the blank configuration illustrated in FIG. 18 will conform rather accurately to that of the pitch surface of the finished thread.

In designing the cross-sectional configuration of a blank for any given size of screw, I prefer to employ the following formulas:

In the above formulas, R equals the radius of curvature 165 of the broad sides such as MP2, 193 and ltl i; r equals the radius of curvature 111 for the lobular portions such as 1%, Hi9 and 11%; C equals the diameter of the circle to which the lobular portions are internal tangents, such as the circle 11 i; and K is the amount the sides of larger radius depart from such a circle, and as indicated by the distance 116. For practical expediency, the fractions, 2.741 and 3.741 may be converted to whole numbers 3 and 4, respectively, for use in designing small size blanks.

In the manufacture of certain products where high precision is not required, or in the case of screws of relatively small size, such as A" and smaller, I have found it feasible to omit the provisions of the smallradius r in the design of the blank as illustrated by FIG. 19. In this case, the sides 12%), 121 and 122 have the same radius of curvature 123 as the corresponding sides in FIG. 18 or in other words the radius 123 is equal in length to the radius 165. I have also found it satisfactory to use the formula in such instances as suggested above. In the case of the blank, the lobular portions will be defined by sides 7.24, 125 and 126 which are concentric about the axis of the blank or, in other words, segments of the circumscribed circle 12.7 which, for the conditions assumed, is the same diameter as the circle 114-. The amount of clearance, K, between the sides 12%, 121 and 122, and the circle 127, indicated at 128 is the same as the clearance 116.

It will be observed that the blank shape illustrated in FIG. 19 conforms very closely to the shape illustrated in PEG. 18 or, in other words, the deviation occasioned by the concentric portions 124, 125 and 126 as distinguished from the portions 108, 109 and 11d of lesser arcuate curvature is so slight as to be hardly noticeable, and especially in the smaller sized screw blanks. After the thread-rolling operation, however, this minor discrepancy disappears entirely in that the cross-sectional shape of the finished screw blank does conform to the shape illustrated in FIG. 18. In other words, in the finished screw, no part of the lobes will be concentric with the screw axis.

To the same extent that the relatively sharp corners between the arcuate sides 12%, 12-1 and 122, and the lobular portions 124, 125 and 126, respectively, disappear in the rolling operation, any other minor deviations from the preferred blank shape as described, also disappear. Therefore, it will be obvious that the various arcuate sides of the blank as illustrated, may be made up of small planar segments, or a combination of arcuate and planar surfaces. By the terms, arcuate lobular configuration, or arcuate polygonal cross section, of substantially uniform width throughout 360 degrees, and the like, as used herein and especially in the claims, it is intended to include all such shapes as approximate an arcuate polygonal configuration so that the blank will roll smoothly between conventional thread-rolling dies as previously described, and still form pronounced lobes in the finished product.

The factor K employed in the above formulas, that is, the maximum amount of clearance between the threads of the screw and of the parent body, is indicated in FiG. 12. In FIG. 12 the extremities of the threads, that is, the crests roots, are illustrated as being perfectly formed but such a condition is seldom true in accordance with accepted commercial thread-rolling practice. Usual- 1y, these extremities are imperfectly formed and, consequently, they do not provide reliable reference points for the determination of the amount of the factor K. It is preferred, therefore, to refer to the pitch diameters and pitch cylinders instead. In FIG. 12 the pitch cylinder for the screw Zti is indicated by the dotted line while the pitch cylinder for the thread formed in the body 51 is indicated by the dotted line 136. The difference in the diameters of the cylinders 135i and 136 may also be represented by the same factor K.

While the value of the factor C in the above formulas is determined by the size of the screw to be provided, the value of the factor K may be varied as desired. The optimum value for the factor K is arrived at as a compromise between two opposing conditions. The greater the amount of K, .the lower will be the frictional resistance between the threads of the screw and the parent body, and hence the lower will be the driving torque. On the other hand, the smaller the amount of K, the greater will be the stripping torque and holding power of the screw. Referring again to FIG. 12, it should be noted thatthe condition illustrated therein represents the unloaded condition of the screw relative to the body 51. FIG. 13 represents the loaded condition, that is, the screw driven home tightly, the opposing forces being represented by the arrows 137 and 138. During such a loaded condition, the lobular portions of the screw will be Brinelled, that is, slightly indented, into the formed threads in the parent body 51 until the flanks 139 of the screw 20 are drawn into substantial engagement with the corresponding adjacent flanks 140 of the body 51. When such engagement occur-s, maximum resistance is provided against the tendency for the threads of either one or the other interengaged elements from stripping. It will also be obvious that the greater the areas of engagement between the flanks 139 of the screw and the adjacent thread flanks 140 of the parent body, the greater will be the stripping resistance.

Obviously, the value selected for the factor K may be varied somewhat depending upon the size of the screw being manufactured and other factors. Through experimentation, I have determined that the following values for K for the most common screw sizes provides an excellent compromise between low driving torque on the one hand, and high stripping torque, or holding power, on the other hand, and with a wide differential therebetween providing an ample range within which automatic clutches for power screw drivers can be adjusted for insuring that the screws may be driven firmly home before disengagement of the clutch and at the same time being assured that disengagement of the clutch will take place before the threads are stripped.

Screw size: Value of K (inches) 2 .0035 3 .004 4 .005 6 .006 8 through 1 .009

I have found it preferable to use a value of K corresponding approximately from one-fourth to one-half of the thread depth in order to obtain a high value of holding power per thread and at the same time provide for low driving torque. It will be obvious that the holding power per thread of the screw will diminish very considerably as the amount of overlap of adjacent threads, as illustrated in FIG. 13, decreases. While the limit is not critical, the amount of overlap at the low points of the sides of the screw should be at least approximately one-third of the thread height.

Stated in another way, for most applications the difference between the distance from the axis to the farthest points on the lobes of the pitch cylinder and the distance from the axis to the nearest points on the sides of the pitch cylinder should not be substantially greater than two-thirds of the thread height. Of course, where high stripping torque is not required, as in the case of the screw described with reference to FIGS. 15 and 16, then the amount of K may be greater.

It will be apparent that after the thread-rolling operation,.the screws of the present invention require no further slotting, milling, or other forming operations, and that they are ready for any heat treating or the like finishing steps. Accordingly, by the rolling operation, a final continuous thread is formed on both the shank and tapered end portions. Assuming, for example, that FIG. 18 represents a typical pitch surface cross section of a screw, shown for example in FIGS. 1, 21, 23 and 25, it will be obvious that the thread extends arcuately across the width of the areas represented by the sides 102, 103

V and 104 with a radius of curvature 105 which is greater than one-half the pitch diameter, i.e., the transverse width of the pitch surface,,the latter being equal to the sum of the radii 105 and 111. As previously mentioned, the pitch diameters of the pitch cylinder do not pass through the'center of the pitch surface throughout 360 degrees. As shown in FIGURE 18, however, all pitch will pass through the point 117, and all pitch diameters measured across the side 104 and lobe 109 will .pass through the point'101. During the thread-rolling operation, the difference between the minimum and maximum radial dimensions of any pitch diameter cross section, or in other words, the difference in the radial distances from the'axial center of the screw to the outermost points of the pitch surface and the innermost points of the pitch surface of any cross section thereof, hereinbefore referred to as the amount K of the thread on both the shank and tapered end portions, is preferably held to an amount not substantially more than two-thirds the depth of the thread on the shank portion. Referring again to FIG. 10, the maximum radial dimension of the pitch surface 66 is indicated, for example, by the distance from point G to the center 60. The minimum radial dimension of the pitch surface 66 is indicated for example, by the distance from point H to the center 60.

In the view shown, the difference in these two dimensions corresponds substantially to the distance 70. As previously indicated, this difference may preferably be held to an amount not substantially more than twothirds, and not substantially less than one-quarter, the depth of the thread.

Referring again to FIG. 18, it will be observed that the peripheral extent of the lobes, 108, 109 and 110, and which may be represented by the linear distance106, respectively, is substantially less than one-half of the extent of each of the sides 102, 103 and 104, and which may be represented by the linear distance 107, respectively. In the case of the illustrated embodiment, the sum of'the linear distances 106 of the three lobes, is approximately equal to twenty-five (25%) percent of the peripheral extent of the pitch cylinder illustrated. It is to be understood that in case of a reduction in the, amount of K, the extent 106 of the lobes is increased with a corresponding reduction in the extent 107 of the intermediate sides 102, 103 and 104. By coincidence, it will be observed that the extent of working engagement of each of the successive lobes at the work-entering end portion of the screw indicated at A, B and C in FIG. 8 is approximately equal to the lobe width as indicated at 106 in FIG. .18. Accordingly, to hold the driving torque of the screw to a relatively low value, I prefer to restrict the linear distance of the lobes 108, 109 and 110 and as represented by the distance 106, toa value less than onehalf the distance 107.

Hereinbef-ore, the cross-sectional shape for all the various modifications of blanks were of arcuate equilateral triangular configuration. It will be obvious that the cross sectional shape may be either five or seven sided. For example, in FIG. 20 is illustrated a five sided cross-sectional configuration. The configuration in 'thisinstance consists of five equiangularly spaced apart sides 142 with intermediate lobe-s 143. In this instance, the radius of curvature of the sides 142 is approximately equal to the diameter of the circumscribed circle 144 while the radius of curvature of the lobes 143, as in previous instances, is less than one-half of such diameter and actually approximately one-fourth thereof. The outline illustrated in FIG. 20 is illustrative of the configuration of the pitch cylinder of the completely rolled threads. For preparation of the blank, the lobular segments between the sides 142 may consist of segments of the circle 144, as in the case of the blank configuration described above with reference to the diagram of FIG. 19. While the poly,,, onal shapes of five or seven sides might be advisable for large size threaded members such as, for example, above /2 in diameter, I prefer to use the three sided polygonal shapes for smaller diameters.

From the description given, it will be obvious that the present invention is adaptable to various different con ventional forms of thread-forming screws. The screw illustrated in FIG. 1 has a thread formation of the vanishing type in that, in its work-entering end portion 25 it is characterized in any axial plane by a constant root diameter and progressively decreasing outside and pitch diameters. r

In the modification illustrated in FIGS. 21 and 22, the screw 145 is provided with a thread formation 146 which is also of the vanishing type in the end portion thereof. The thread in the work-entering end portion 147 is further characterized, in any axial plane, by decreasing root, pitch and outside diameters. The pitch cylinder of the thread formation on the shank is indicated by the dotted lines 181; The pitch cone of the work-entering end portions is indicated by the dotted lines 182. It will be noted that the inwardly tapering sides of the cone, as viewed in longitudinal section, are curved rather than straight. Both the pitch cylinder 181 and the pitch cone 182 are of lobular cross-sectional configuration, as is readily apparent from the end View of FIG. 22, and moreover, every transverse cross section of such pitch surfaces is of equal transverse width throughout 360 degrees.

In FIGS. 23 and 24 is illustrated a screw 149 having a thread formation 150 which is finished or completely formed in its main shank portion but which is unfinished in its work-entering end portion 151 where the thread formation in any'axial plane is characterized by a constant root and pitch diameter and a decreasing outside diameter. The pitch cylinder of the thread formation 150 and 151 is indicated by dotted lines 184. The pitch cylinder, as is apparent from the end view of FIG. 24, is of lobular cross-sectional configuration and is of equal transverse width throughout 360 degrees and throughout the entire length of the shank and work-entering end portions. Notwithstanding the fact that the thread crests are unfinished in the work-entering portion, the peripheral contour of at least the last thread of such portion conforms substantially to the configuration illustrated and described with reference to FIGS. 8 and 9. It should be emphasized, however, that insofar as the work-entering end portion is concerned, it is immaterial as to whether the crests of the threads are finished particularly over the extent of the sectors D, E and F as indicated in FIG. 8. Such unfinished thread crest portious are also indicated at 152 in MG. 24. it is important, however, that the radial extent of the thread portions increase gradually over the sectors A, B and C smoothly, and as further shown at 153 in PEG. 24.

In FIGS. 25 and 26 is illustrated a further form of screw 155 which is provided with a finished thread over its main shank portion and an unfinished thread over its work-entering end portion 157. This screw, in any axial plane, in its work-entering end portion 157, has a constant pitch diameter while the root diameter increases in the direction toward the outer end 158 while the outside diameter progressively decreases toward the same outer end. The pitch cylinder of the thread formation throughout the entire length thereof is indicated by dotted lines 186. This pitch cylinder is of l-obular cross-sectional configuration and is of equal transverse width throughout 360 degrees throughout both the shank and work-entering end portions. The device in this instance is formed through the use of contoured thread-rolling dies in conjunction with an untapered blank, that is, a blank of lobular cross section but of uniform transverse width throughout its entire length. This screw is of the so-called captive type in that when it is threaded into a pilot hole having a diameter only slightly larger than the extremity 158 of the screw, the female thread formed in the parent body increases progressively at a rate comparable to the decrease in the root diameter of the threa of the screw so that the fully for-med thread in the parent body has an internal crest diameter which is smaller than the diameter of the screw end 158.

Reference should also be made to my copending divisional application Serial Number 220,283 filed August 29, 1962, in which the self tapping screws disclosed here- A in are claimed.

It is to be understood that while the present invention has been described with particular reference to threadforming screws, it is obvious that the invention is not to be necessarily so limited in that it is applicable to any other device adapted for forming its own female threads in a parent body. Accordingly, it is intended in the following claims to cover all such variations and modifications as fall within the true spirit and scope of the invention.

I claim:

1. The method of making a lobular thread forming fastener device having a tapered work entering end, which comprises the steps:

(a) forming a blank by working an odd number of uniformly circumferentially spaced apart surface areas extending longitudinally of a length of metal stock inwardly of said length to a greater extent than the intermediate surface areas and thereby producing a shank portion which is of generally arcuate polygonal cross-section of substantially uniform width throughout 360 degrees,

(b) and then circumferentially reworking the end of said shank portion gradually and increasingly inwardly in the direction toward the extremity of said end portion while maintaining an arcuate polygonal cross-sectional configuration substantially concentric and symmetrical with that of said shank portion to form a tapered end on said blank portion,

(c) and then generating on a centerless basis a continuous thread formation to its final shape on both said shank and tapered end portions having pitch diameter cross-sections substantially the same configuration as that of the corresponding cross-sections of said blank by simultaneously rolling both said id shank and tapered portions of said blank between conventional, uniformly spaced apart thread rolling dies,

(d) and by said rolling operation forming said thread on both said shank and tapered end portion which extends arcuately across the width of said first mentioned areas with a radius of curvature greater than one-half the pitch diameter and maintaining the difference between the minimum and maximum radial dimensions of any pitch diameter cross section of such thread on said shank and tapered end portions to an amount not substantially more than two-thirds the depth of the fastener thread on said shank portion.

2. The method of making a lobular thread forming fastener device having a tapered work entering end, which comprises the steps:

(a) forming a blank by working an odd number of uniformly circumferentialiy spaced apart surface areas extending longitudinally of a length of metal stock inwardly of said length by an amount such that the difference in the radial distance from the axis of such length to the midpoint of such areas and the corresponding radial distance to the midpoint of the intermediate surface areas is not substantially more than two-thirds and not substantially less than one-quarter the depth of the fastener thread, whereby to form a shank portion which is of generally arcuate polygonal cross-section of substantially uniform width throughout 360 degrees,

(b) and then circumferentially reworking the end of said shank portion gradually and increasingly inwardly in the direction toward the extremity of said end portion while maintaining an arcuate polygonal cross-sectional configuration concentric and symmetrical with that of said shank portion to form a tapered end of said blank portion,

(c) and then generating on a centerless basis a continuous thread formation to its final shape on both said shank and tapered end portions having pitch diameter cross-sections substantially the same as the corresponding cross-sections of said blank by simultaneously rolling both said shank and tapered portions of said blank between conventional, uniformly spaced apart thread rolling dies,

(d) and by said rolling operation forming said thread on both said shank and tapered end portions which extend arcuately across the width of said first mentioned areas with a radius of curvature greater than one-half the pitch diameter and maintaining the difference between the minimum and maximum radial dimension of any pitch diameter cross section of such thread on said shank and tapered end portions to an amount not substantially more than two-thirds the depth of the fastener thread on said shank portion.

3. The method according to claim 2 in which the blank is formed by forcing at least a portion of a length of round metal stock through a reshaping die to produce the arcuate polygonal cross-sectional shape.

4. The method according to claim 2 in which the blank is inwardly worked on three uniformly circumferentially spaced apart areas to produce an arcuate triangular crosssectional configuration on both the shank and tapered end portions.

5. The method according to claim 2 in which said odd number of areas are each inwardly worked over a greater circumferential width than that of the intermediate areas.

6. The method according to claim 5 in which said odd number of areas are inwardly worked to provide said areas with a radius of curvature greater than one-half the diameter of said metal stock and the intermediate areas inwardly worked to provide the same with a radius of curvature less than one-half the diameter of said metal stock.

7. The method according to claim 2 in which the round metal stock is drawn through a reshaping die so as to form said odd number of areas and then individual workpieces severed therefrom for further processing.

8. The method of making a lobular thread forming device having a tapered work entering end comprising the steps:

(a) reworking a length of metal stock of circular cross section to form a blank by applying reforming pressures to an odd number of circumfercntially spaced apart areas extending longitudinally of said length of stock producing similar surfaces on each of said areas which in cross-section are arcuately curved with a radius of curvature greater than one-half the diameter of the reformed part, the circumferential width of said areas being greater than that of the intermediate. areas, and the reforming pressures being applied by an amount such that the difference in the distance from the axis of the part to the center of first mentioned areas and to the center of the intermediate areas is not substantially more than two-thirds and not substantially less than one-quarter the depth of the fastener thread, whereby to form a shank portion which is of arcuate polygonal cross-section of substantially uniform width throughout 360 degrees,

(b) and then reforming an end portion of said shank portion gradually and increasingly inwardly in the direction toward the extremity of said end portion producing a tapered end on said blank portion while maintaining thereon an arcuate polygonal cross-sectional configuration substantially concentric and symmetrical with that of said shank portion, 7

(c) and then generating on a centerless basis a continuous thread formation to its final shape on both said shank and tapered end portions having pitch diameter cross sections generally similar to the corresponding cross sections on said blank by simultaneously rolling both said shank and tapered end portions of said blank between uniformly spaced apart thread rolling means, 7

(d) and by said rolling operation forming said thread on both said shank and tapered end portion which extends arcuately across the width of said first mentioned areas with a radius of'curvature greater than one-half the pitch diameter and across the narrower Width of said intermediate areas with a radius of curvature less than one-half the pitch diameter, the thread portions on one area merging smoothly with the thread portions of adjacent areas, and simultaneously maintaining the difference between the minimum and maximum radial distances of any pitch diameter cross section of the thread on at least said shank portion to an amount not substantially more than two-thirds and not substantially less than onequarter the depth ofthe thread on said shank portion.

9. The method of making a lobular thread-forming screw comprising the steps: 1

(a) drawing a length of round metal stock through a forming die and thereby working an odd number of -uniformly circumferentially spaced apart surface areas extending longitudinally of said length inwardly thereof to a greater extent than the intermediate surface areas, and thereby producing stock of generally arcuate polygonal cross section of substantially uni form width throughout 360 degrees and having an odd number of sides, r

(b) severing a workpiece from said length, (c) upsetting one end portion of said workpiece and thereby forming a blank having a driving head and a shank,

(d) and during said upsetting step simultaneously re- (e) then simultaneously rolling a continuous thread formation to its final shape on both said shank and said work entering end having pitch diameter cross sections that are substantially similar in shape to the corresponding cross sections of said shank and said work entering end of said blank.

10. The method of making a lobular thread-forming device,

(a) forming a blank having a Work-entering end portion,

(b) shaping said portion into an arcuate lobular configuration having an odd number of arcuate sides with each transverse cross section having substantially equal width throughout 360 degrees whereby said portion will roll between conventional, uniformly spaced apart thread-rolling dies,

(c) rolling into its final shape a continuous thread on said blank including said end portion,

' (d) and while rolling said thread on said end portion,

maintaining thereon an arcuate lobular pitch surface configuration which in every transverse cross section is of equal transverse Width throughout 360 degrees, and simultaneously by said rolling operation producing a taper on the crest of the continuous thread into its final shape on said end portion, which taper extends inwardly and toward the tip of said end portion.

References Qiterl by the Examiner UNITED STATES PATENTS 1,832,167 11/31 Wilcox Q 1010 1,912,517 6/33 De Lapotterie 10152 1,987,474 1/35 Grant 8547' 2,017,341 10/35 Cummins l0'-271 2,314,391 3/43 De Vellier 10-10 2,352,982 7/44 Tomalis 85-47, 2,526,802 10/50 Carlile 29-190 2,562,516 7/51 Williams -61 2,679,774 6/54 MacDonald 8061 2,764,804 10/56 Arness 29-190 2,807,813 10/57 Welles 10-l52 2,846,056 V 8/58 Hampton 10-27 FOREIGN PATENTS 4,271 12/74 Great Britain. 223,231 6/56 Japan.

Examiners. 

1. THE METHOD OF MAKING A LOBULAR THREAD FORMING FASTENER DEVICE HAVING A TAPERED WORK ENTERING END, WHICH COMPRISES THE STEPS: (A) FORMING A BLANK BY WORKING AN ODD NUMBER OF UNIFORMLY CIRCUMFERENTIALLY SPACED APART SURFACE AREAS EXTENDING LONGITUDINALLY OF A LENGTH OF METAL STOCK INWARDLY OF SAID LENGTH TO A GREATER EXTENT THAN THE INTERMEDIATE SURFACE AREAS AND THEREBY PRODUCING A SHANK PORTION WHICH IS OF GENERALLY ARCUATE POLYGONAL CROSS-SECTION OF SUBSTANTIALLY UNIFORM WIDTH THROUGHOUT 360 DEGREES, (B) AND THEN CIRCUMFERENTIALLY REWORKING THE END OF SAID SHANK PORTION GRADUALLY AND INCREASINGLY INWARDLY IN THE DIRECTION TOWARD THE EXTREMITY OF SAID END PORTION WHILE MAINTAINING AN ARCUATE POLYGONAL CROSS-SECTIONAL CONFIGURATION SUBSTANTIALLY CONCENTRIC AND SYMMETRICAL WITH THAT OF SAID SHANK PORTION TO FORM A TAPERED END ON SAID BLANK PORTIONS, (C) AND THEN GENERATING ON A CENTERLESS BASIS A CONTINUOUS THREAD FORMATION TO ITS FINAL SHAPE ON BOTH SAID SHANK AND TAPERED END PORTIONS HAVING PITCH DIAMETER CROSS-SECTIONS SUBSTANTIALLY THE SAME CONFIGURATION AS THAT OF THE CORRESPONDING CROSS-SECTIONS OF SAID BLANK BY SIMULTANEOUSLY ROLLING BOTH SAID SHANK AND TAPERED PORTIONS OF SAID BLANK BETWEEN CONVENTIONAL, UNIFORMLY SPACED APART THREAD ROLLING DIES, (D) AND BY SAID ROLLING OPERATION FORMING SAID THREAD ON BOTH SAID SHANK AND TAPERED END PORTION WHICH EXTENDS ARCUATELY ACROSS THE WIDTH OF SAID FIRST MENTIONED AREAS WITH A RADIUS OF CURVATURE GREATER THAN ONE-HALF THE PITCH DIAMETER AND MAINTAINING THE DIFFERENCE BETWEEN THE MINIMUM AND MAXIMUM RADIAL DIMENSIONS OF ANY PITCH DIAMETER CROSS SECTION OF SUCH THREAD ON SAID SHANK AND TAPERED END PORTIONS TO AN AMOUNT NOT SUBSTANTIALLY MORE THAN TWO-THIRDS THE DEPTH OF THE FASTENER THREAD ON SAID SHANK PORTION. 